Optical communication technologies using light of a visible light range have recently been attracting much interest. Along with the rapid proliferation of lighting devices using light emitting devices such as Light Emitting Diodes (LEDs) or the like, techniques for realizing more convenient, higher-rate data communication using infrastructures including indoor and outdoor lightings are under development.
Although LEDs are most promising for light emission in high-speed optical data transmission in terms of influence on the human body and medical equipment, other semiconductor light emitting devices are also available, inclusive of Laser Diodes (LDs) or Super Luminescent Diodes (SLDs) that offer a faster response performance. Also, data rate is dependent on the response speed of a light emitting device in optical communication. In this context, a technique for transmitting a large amount of data stably in one signal of light emission of a light emitting device is needed.
In relation to the above problem, for example, Japanese Laid-Open Patent Application Publication No. 2004-147063 (Patent Document 1) discloses a structure for adjusting a control voltage to change the intensity of light emitted from a light source according to a frequency-multiplexed signal. With the use of this structure, it is possible to change the light intensity of a light source or transmit multiplexed data under on/off control. Compared to non-multiplexing, a higher data rate can be achieved. Japanese Laid-Open Patent Application Publication No. 2006-325085 (Patent Document 2) discloses a structure for transmitting different data by controlling light emission of a plurality of light sources having different spectrums. To be more specific, Patent Document 2 discloses a structure for transmitting data by mapping digital values (1, 0) to phase changes at a transition timing of a Pulse Width Modulation (PWM) signal.
However, the transmission technique of Patent Document 1 increases a transmission error rate due to a decrease in light intensity on a transmission path because multiplexed data is transmitted by modulating a light intensity. Especially in visible light communication, if a light source as a transmission apparatus is a lighting device, its light intensity is decreased due to effects of shielding. Consequently, an increased transmission error rate decreases a data rate. Even though the transmission technique of Patent Document 1 is implemented, it is not easy to increase a data rate up to the response limit of the light source.
Meanwhile, no more than a few-fold increase in data rate is expected from the transmission technique of Patent Document 2 because the multiplicity of data is equal to or less than the number of light sources having different emission spectrums. Moreover, the multiplicity is further decreased, except when a light source emits white light.
The present invention is intended to address the above problems. Accordingly, an object of the present invention is to provide a visible light transmission apparatus, a visible light reception apparatus, a visible light communication system, and a visible light communication method, for enabling transmission of data modulated to a multiple-value number including as many values as or values more than number of light sources and at the same time, suppressing an increase in a transmission error rate, caused by a decrease in light intensity on a transmission path.